The invention relates to a picture display cell which comprises a liquid crystalline compound provided between two substrate plates provided on their inner surfaces with an electrode and an orientation layer which gives the liquid crystalline compound a homeotropic orientation.
The surface of the inner wall of the substrate employed in such a display cell must comprise hydroxyl groups and be provided, for example, with a layer of quartz (SiO.sub.2) or a metal oxide, for example, TiO.sub.2 over the electrode(s). The substrate is usually manufactured from glass and on its inner surface has a layer of SiO.sub.2 provided over the electrode(s) by means of a chemical vapour deposition process.
Such a picture display cell has been used frequently in a variety of apparatuses and products in the past few years.
For various types of liquid crystal displays, a homeotropic orientation of the liquid crystalline compound with respect to the substrate is necessary. With a homeotropic orientation the molecules of the liquid crystalline compound have a preferential position which is perpendicular to the surface of the substrate. A homeotropic orientation is necessary inter alia for liquid crystal displays with dynamic scattering, ECB (electrically controlled birefringence) and cholesteric-nematic phase transitions.
In order to obtain a homeotropic orientation, the substrate is often treatedwith a bi- or trifunctional silane compound. There are compounds which per molecule have two or three active atoms or groups which can react with the substrate and thus produce a bonding. Examples of such compounds are methyltrichlorosilane and methylphenyldichlorosilane. The chlorine atoms react with the hydroxyl groups present on the substrate. Instead of chlorosilane compounds, alkoxysilane compounds may also be used, for example, dimethylbisethoxysilane and methyltriethoxysilane. These substances are less reactive than the chorosilanes and must be employed in an aqueous medium in which the alkoxy groups are converted into hydroxyl groups which react with the hydroxyl groups on the substrate.
The disadvantage of the use of the above-mentioned bi- and trifunctional silanes is that the surface of the resulting orientation layer is not accurately defined, so that the orientation of the liquid crystalline compound is not optimal. The poor definition of the orientation layer in applicants' opinion is caused by polymerisation taking place in the presence of moisture. As a result of the polymerisation, the surface of the orientation layer is not uniform. Moreover, active OH-groups are created in the orientation layer and from adsorption sites for constituents of the layer of the liquid crystalline compound.